The exhaust gas of engines contains nitrogen oxide (NOx). An exhaust gas post-treatment device is disposed at an intermediate position in an exhaust path for inhibiting emission of the nitrogen oxide into the air. The exhaust gas post-treatment device includes a filter for trapping particulate substance in the exhaust gas, a urea aqueous solution mixing device disposed on the exhaust downstream side of the filter in order to add a urea aqueous solution to the exhaust gas, and a reducing catalyst disposed on the further downstream side of the mixing device in order to reduce and purify nitrogen oxide in the exhaust gas.
The urea aqueous solution mixing device in the aforementioned exhaust gas post-treatment device includes an injector that injects a urea aqueous solution as a reducing agent into an exhaust pipe. The urea aqueous solution, injected into the exhaust pipe from the injector, is mixed with the exhaust gas and is supplied to the reducing catalyst on the exhaust downstream side. Here, firstly, the urea aqueous solution is thermally decomposed by the exhaust gas and ammonia is thereby obtained. Then, in the reducing catalyst, the ammonia is used as a reducing agent and the nitrogen oxide in the exhaust gas is reduced and purified.
In such device, a sufficient reaction time is required for thermally decomposing the urea aqueous solution and thereby Obtaining ammonia. To achieve this, it is required to reliably produce a long distance between the injector and the reducing catalyst. When the distance is short, the urea aqueous solution injected within the exhaust pipe is not sufficiently mixed with the exhaust gas, while partially attaching as droplets to the inner wall of the exhaust pipe cooled by the external air. The urea aqueous solution, thus attached to the inner wall of the exhaust pipe, is not easily decomposed. Therefore, there may be deficiency in ammonia to be supplied to the reducing catalyst. Further, the urea aqueous solution, changed into droplets on the inner wall of the exhaust pipe, may be crystallized and deposited on the inner wall.
In view of the above, as described in Publication of Japanese Translation of PCT International Application No. JP-A-2008-509328 and WO2006/025110 A1, devices have been provided that a double-nested pipe structure is produced by disposing an inner pipe within an exhaust pipe on the exhaust downstream side of an injector and a urea aqueous solution is configured to be injected into the inner pipe. The exhaust gas herein flows around the outer peripheral part of the inner pipe. Therefore, the inner pipe is heated by the exhaust gas, and accordingly, the urea aqueous solution can be inhibited from attaching as droplets to the inner wall of the inner pipe,